RETARDATION OF MAMMARY INVOLUTION IN MICE BY INJECTION OF LACTOGENIC HORMONE1

Abstract

Of 21 lactating mice isolated from their young on the 4th day post-partum, 12 were given 7 daily injs. of 20-60 I.U. of prolactin and 9 were untreated. Compared with the untreated controls, 10 of the animals receiving prolactin exhibited definite retardation in mammary involution. The avg. involution in the treated animals was that of 4 days in contrast to the 7 days of segregation from their young. The inhibition of involution manifested itself chiefly as maintenance of the lobule-alveolar and duct systems in a condition very similar to that at parturition. Sufficient secretion was present to indicate partial retention of secretory activity. The degree of retardation appeared independent of the dose; and ovariectomy was without influence upon the regression in the treated animals. Suckling evidently prevents mammary regression and maintains lactation in mice by virtue of stimulation of the nipples eliciting release of lactogenic hormone which, in turn, maintains the mammary glands.